A natural resource such as oil or gas residing in a subterranean formation can be recovered by drilling a well into the formation. In particular, a wellbore is typically drilled down to the subterranean formation while circulating a drilling fluid through the wellbore. After the drilling is terminated, a string of pipe, e.g., casing, is run in the wellbore. Primary cementing is then usually performed whereby a cement slurry is pumped down through the string of pipe and into the annulus between the string of pipe and the walls of the wellbore to allow the cement slurry to set into an impermeable cement column and thereby seal the annulus. Further drilling into the subterranean formation followed by cementing is continued until the production zone containing the natural resource such as oil or gas is reached. The purpose of cementing different subterranean zones prior to reaching the production zone is to isolate the flow of fluids from the cemented zones into the wellbore. Frequently this process is referred to as zonal isolation. Subsequent secondary cementing operations, i.e., any cementing operation after the primary cementing operation, may also be performed. One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is forced under pressure to areas of lost integrity in the annulus to seal off those areas.
As a wellbore is being prepared to provide for the recovery of oil or gas residing in the subterranean formation, permeable zones through which fluid can undesirably migrate often need to be plugged. For example, fractures previously created in the formation to increase the production of the oil or gas may eventually produce water rather than oil or gas. To provide for the production of more oil or gas, a fracturing fluid may again be pumped into the formation to form additional fractures therein. However, the previously used fractures first must be plugged to prevent the loss of the fracturing fluid into the formation via those fractures. Thus, plugging compositions are also useful in stimulation of new production zones after the prevailing production zone stimulated via perforations through the casing has become depleted. In such cases the existing perforations need to be plugged so that the fracturing fluid can be diverted to new perforations in a new zone.
In addition to the fracturing fluid, other fluids used in servicing a wellbore may also be lost to the subterranean formation while circulating the fluids in the wellbore. In particular, the fluids may enter the subterranean formation via permeable zones such as depleted zones, zones of relatively low pressure, lost circulation zones having naturally occurring fractures, weak zones having fracture gradients exceeded by the hydrostatic pressure of the servicing fluid, and so forth. As a result, the service provided by such fluid is more difficult to achieve. For example, a drilling fluid could be lost to the formation, resulting in the circulation of the fluid in the wellbore being too low to allow for further drilling of the wellbore. Also, a cement/sealant composition could be lost to the formation as it is being placed in the wellbore, thereby rendering the cementing/sealing operation ineffective in maintaining isolation of the formation. The loss of such fluids increases the cost of the overall operation due to the prolonged rig time required, the fluids being relatively expensive, and possibly a need to install casing. The plugging of such permeable zones in the formation with plugging compositions is thus needed to avoid such problems.
Permeable zones in the subterranean formation may further allow water to undesirably flow from the formation into the wellbore during primary and secondary cementing operations, resulting in the influx and crossflows of water through cement slurries that have been placed in the wellbore. The influx of water may occur during a transition phase in which a cement slurry changes from a true hydraulic fluid to a highly viscous mass showing some solid characteristics. Consequently, flow channels may remain in the cement slurry after it has completely set. Those flow channels could allow the water to flow from one subterranean zone to another such that zonal isolation is no longer achieved. Further, the water can intermix with and dilute the cement slurry, causing deterioration of the cement properties such as its density, its final compressive strength, and its rheology. As such, the permeable zones in the formation need to be plugged to prevent the in flux and crossflows of water through the cement slurry before it sets.
The isolation of the subterranean formation also may be compromised by permeable zones present in the annulus and/or the wall of the casing in the wellbore. Permeable zones such as voids in the cement column placed in the annulus may result from an incomplete fill in of the annulus during primary cementing. Permeable zones may also be caused by the expansion and contraction of the casing and the cement column due to cyclical changes in underground pressures and temperatures during the life of the well. Moreover, the cement column may experience impacts and shocks generated by subsequent drilling, pressure testing, hydraulic fracturing, or other well operations. Unfortunately, conventional cement suffers from the drawback of being brittle and fragile and thus often cannot sustain such stress. Consequently, cracks or voids may form in the cement column and/or the casing wall. Further, microannuli may form between the cement column and the casing and between the cement column and the subterranean formation. The microannuli, the voids or cracks in the cement column, and/or the voids or cracks in the casing wall need to be plugged to ensure that the subterranean formation remains isolated from other subterranean formations.
A variety of compositions have been developed for plugging the permeable zones described above, and such compositions may form a sealing mass having a relatively high viscosity, for example, when contacting water in the wellbore or when two fluids are allowed to contact with each other near the zone requiring plugging. The use of such plugging compositions, however, does not necessarily ensure that fluid will be prevented from flowing through the permeable zones in which they are placed. The plugging compositions may be unable to withstand the pressures applied by the fluids in the wellbore or the formation fluids due to, for example, an insufficient amount of friction between the compositions and adjacent surfaces in the wellbore. When subjected to such pressures, the plugging compositions thus may be dislodged from their positions in the permeable zones. Further, the plugging compositions may lack the the ability to resiliently conform to the changes in the sizes and shapes of the permeable zones that often occur during the follow up operations in the wellbore in which they are used. Therefore, fluid may undesirably leak through areas of the permeable zones where the plugging compositions have debonded from the perimeters of those zones. Accordingly, a need exists to develop methods for improving the ability of plugging compositions to effectively seal permeable zones.